The Critique of Pure Reason (Kritik der reinen Vernunft), first published in 1781 with a second edition in 1787, is widely regarded as the most influential and widely read work of the German philosopher Immanuel Kant and one of the most influential and important in the entire history of Western philosophy. It is often referred to as Kant's "first critique", and was followed by the Critique of Practical Reason and the Critique of Judgment.

Regarded as a ground-breaking work in Western philosophy, Kant saw the first critique as an attempt to bridge the gap between rationalism and empiricism and, in particular, to counter the radical empiricism of David Hume.

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Kant's rejection of Hume's empiricism

Hume's conclusions, Kant realized, rested on the premise that all ideas are representations of sensory experience (known as Ideal theory of the mind). The problem that Hume identified was that basic principles like cause and effect cannot be derived from sense experience only. Kant's goal, then, was to find some way to derive cause and effect without relying on empirical knowledge. Kant rejects analytical methods for this, arguing that analytic reasoning can't tell us anything that isn't already self-evident. Instead, Kant argued that we would need to use synthetic reasoning. But this posed a new problem—how can one have synthetic knowledge that is not based on empirical observation—that is, how can we have synthetic a priori truths.

Kant argued that there are synthetic a priori truths. He reasoned that statements such as those found in geometry and Newtonian physics are synthetic a priori knowledge and wanted to establish how this could be possible. This also led him to inquire whether it could be possible to ground synthetic a priori knowledge for a study of metaphysics, because most of the principles of metaphysics from Plato through Kant's immediate predecessors made assertions about the world or about God or about the soul that were not self-evident but which could not be derived from empirical observation. This led to his most influential contribution to metaphysics: the abandonment of the quest to try to know the world as it is "in itself" independent of our sense experience. He demonstrated this with a thought experiment, showing that we cannot meaningfully conceive of an object that exists outside of time and has no spatial components and isn't structured in accordance with the categories of the understanding, such as substance and causality. Although we cannot conceive of such an object, Kant argues, there is no way of showing that such an object does not exist. Therefore, Kant says, metaphysics must not try to talk about what exists, but instead about what is experienced and how it is experienced.

This provided Kant with the basis to distinguish between phenomena, things as they appear to our senses, and noumena, things that are purely objects of thought independently of sense perception, which, by definition, we can never experience; in Kant's words, "thoughts without content [are] empty, and intuitions without concepts [are] blind". The phenomenon is only the representation of the noumenon that a person receives through their sense organs, which their mind then structures in accordance with the categories of the understanding. Kant's whole metaphysical system, which is based on the operations of cognitive faculties, was meant to describe the world as we experience it—a much more modest task than describing the world as it is beyond our experience of it, which, according to Kant, is what all previous philosophy was mistakenly trying to do.

Kant termed his critical philosophy "transcendental idealism", which for him is intimately linked with empirical realism. That is, our empirical knowledge is real, but it cannot know what transcends the operations of our cognitive faculties. Transcendental idealism describes Kant's method of seeking the conditions of the possibility of our knowledge of the world and recognizes that there are things that transcend the limits of our cognitive faculties. It is because of taking into account the role of our cognitive faculties in structuring the known and knowable world that in the second preface to the "Critique of Pure Reason" Kant compares his critical philosophy to Copernicus' revolution in astronomy. Kant writes: "Hitherto it has been assumed that all our knowledge must conform to objects. But all attempts to extend our knowledge of objects by establishing something in regard to them a priori, by means of concepts, have, on this assumption, ended in failure. We must therefore make trial whether we may not have more success in the tasks of metaphysics, if we suppose that objects must conform to our knowledge" [Bxvi]. Just as Copernicus revolutionized astronomy by changing the point of view and taking the position of the observer into account, Kant's critical philosophy takes into account the position of the knower of the world in general and reveals its impact on the structure of his known world.

Kant's transcendental idealism should be distinguished from idealistic systems such as Berkeley's. While Kant claimed that phenomena depend upon the conditions of sensibility, space and time, and on the synthesizing activity of the mind manifested in the rule-based structuring of perceptions into a world of objects, this thesis is not equivalent to mind-dependence in the sense of Berkeley's idealism. For Berkeley, something is an object only if it can be perceived. For Kant, on the other hand, perception does not provide the criterion for the existence of objects. Rather, the conditions of sensibility (space and time) and the categories of the understanding provide the "epistemic conditions", to borrow a phrase from Henry Allison, required for us to know objects in the phenomenal world.

Kant's approach

Title page of the 1781 edition.

The Critique of Pure Reason is an attempt to answer two questions: "What do we know?" and "How do we know it?".

Kant approaches the questions by looking at the relationship between knowledge based on reason (what we know purely logically, prior to or independently of experience, or a priori) and knowledge based on experience (what we know based on the input of our senses or a posteriori).

In Kant's view, a priori intuitions and concepts provide us with some a priori knowledge, which also provides the framework for our a posteriori knowledge. For example, Kant argues that space and time are not part of what we might regard as objective reality, but are part of the apparatus of perception, and causality is a conceptual organizing principle that we impose upon nature.

In other words, space and time are a form of seeing and causality is a form of knowing. Both space and time and our conceptual principles and processes pre-structure our experience.

When we see a box as three-dimensional, the shape of the box may not be part of the box's nature. Kant argues that the spatio-temporal aspect of our perception of the shape of the box comes from us, in interaction with the box, not just from the box itself. When we experience events as causing other events, it is because we have a concept of causality in nature into which we fit our experience.

Things as they are "in themselves" are unknowable. For something to become an object of knowledge, it must be experienced, and experience is prestructured by the activity of our own minds -- both space and time as the forms of our intuition or perception, and the unifying, structuring activity of our concepts. These two aspects of our minds turn things-in-themselves into the world of our experience. We are never passive observers or knowers.

Kant's I—the Transcendental Unity of Apperception—is similarly unknowable. I am aware that there is an "I", subject, or self that accompanies all of my experience and consciousness. But since I only experience it in time, which is a "subjective" form of perception, I can never know directly that "I" that is appearing in time as it might be "in itself", outside of time. Thus we can never truly know ourselves as we might be outside of or prior to the forms through which we perceive and conceive ourselves.

Transcendental Doctrine of Elements

Transcendental Aesthetic

Kant separates the mind into two faculties; intuition and understanding. The Transcendental Aesthetic is that part of the CPR that considers the contribution of intuition to our knowledge or cognition. In discussing intuition Kant says: "In whatever manner and by whatever means a mode of knowledge may relate to objects, intuition is that through which it is in immediate relation to them" (A19/B33). Intuition is responsible for providing the mind with objects, by way of "appearances".

Kant then goes on to distinguish between the matter and the form of appearances. The matter is "that in the appearance which corresponds to sensation" (A20/B34). The form is "that which so determines the manifold of appearance that it allows of being ordered in certain relations" (A20/B34). Kant's revolutionary claim is that the form of appearances — which he later identifies as space and time — is a contribution made by the faculty of intuition to cognition, rather than something that exists independently of the mind. This is the thrust of Kant's doctrine that space and time are transcendentally ideal.

Kant's arguments for this conclusion are widely debated amongst Kant scholars. Some see the argument as based on Kant's conclusions that our representation of space and time is an a priori intuition. From here Kant is thought to argue that our representation of space and time as a priori intuitions entails that space and time are transcendentally ideal (see Henry Allison, "Kant's Transcendental Idealism"). Others see the argument as based upon the question of whether synthetic a priori judgments are possible. Kant is taken to argue that the only way synthetic a priori judgments, such as those made in geometry, are possible is if space is transcendentally ideal.

Space and time

Kant gives two expositions of space and time: metaphysical and transcendental. The metaphysical expositions of space and time are concerned with clarifying how those intuitions are known independently of experience. The transcendental expositions attempt to show how the metaphysical conclusions might be applied to enrich our understanding.

The first metaphysical expositions unfold by describing what both space and time really are. The argument proceeds according to the following five main points:

Space and time are not in themselves general concepts; rather, they are intuitions. Space is not in itself a concept because one can imagine things external to them, and this feat of the imagination supposes a prior understanding of space. Therefore the internal representation of space can't be drawn from experience by the acquaintance with external sensations and establishing relations between them; rather, experiences with the external are themselves impossible unless they presuppose the ability to understand space. Similarly, the perception of co-existence and succession could not occur without first having an understanding of time.

Space is a necessary representation that is the foundation of all external experiences. We can never imagine anything without space. Time is also a necessary representation, but in a sense that is more powerful, since it underlies every intuition whatsoever.

For space, this a priori necessity is the foundation of philosophical certainty of all geometrical principles and the possibility of their a priori construction. As far as time is concerned, by the same a priori necessity we may also find the possibility of philosophical principles concerning time and its axioms.

Neither space nor time are general concepts. In effect, we can't initially imagine anything but one, unitary space, and, when we talk about many spaces, we mean by that that those sub-parts occupy part of the same, unique space. The same reasoning forces the same conclusion for time: different times are just part of the same time. Moreover, the fact that space and time are not concepts can be demonstrated in that space and time are necessary and universal conditions for experience, and so, must be intuitions.

Finally, space and time are both infinite (though this is not strictly true, as he explains later in the 'Antinomy of Pure Reason'.) The representation of space is infinite in the sense that "all the parts of space coexist ad infinitum" (p.70). Time is similarly unlimited, in that "every determinate magnitude of time is possible only through limitations of one single time that underlies it" (p. 75).

In the transcendental exposition, Kant returns to what he demonstrated in his metaphysical exposition in order to show that the sciences would be impossible if space and time were not kinds of pure a priori intuitions. He asks the reader to take the proposition, "two straight lines can neither contain any space nor, consequently, form a figure", and then to try to derive this proposition from the concept of a straight line and the number two. He concludes that it is simply impossible. Thus, as we can't obtain this information from analytic reasoning; so it must be by way of synthetic reasoning, i.e., a synthesis of concepts (in this case two and straightness) with the pure (a priori) intuition of space.

But in this case, it wasn't experience that furnished the third term; otherwise, we would lose the necessary and universal character of geometry (though bear in mind that the universality and necessity of Euclidean geometry is taken for granted by Kant). Since only space, which is a pure a priori form of intuition, makes this synthetic judgment, it must then be an a priori. If geometry doesn't serve this pure a priori intuition, it is empirical, and would be an experimental science. But geometry doesn't proceed by measurements -- it proceeds by demonstrations.

Kant rests his demonstration of the apriority of space on the example of geometry. He reasons that therefore if something exists, it needs to be intelligible. If we attacked this argument, we would doubt the universality of geometry (which no honest person would do, in Kant's estimation).

The other part of the transcendental aesthetic argues that time is a pure a priori intuition which renders mathematics possible. As with space, time is not a concept, since otherwise it would obey formal logical analysis (and therefore, of the principle of non-contradiction). However, time makes it possible to deviate from the principle of non-contradiction: indeed, it is possible to say that A and non-A are in the same place if one considers them in different times, and a sufficient alteration between states were to occur. Time and space cannot thus be regarded as existing beings in themselves. They are a priori forms of sensory intuition. Nothing can ever be found in an experiment which cannot be attributed to a time and a place.

The current interpretation of Kant is that he is declaring that the subject contributes the preconditions for experience of space and time. The Kantian thesis was to say that in order for the subject to have any experience at all, they have an experience of these forms. Some scholars have offered this position as an example of psychological nativism, as a rebuke to some aspects of classical empiricism.

Kant's thesis is restricted to appearances (or phenomena), and it necessarily implies that we cannot know the things in themselves (or noumena). This is where the paradox of Kantian philosophy resides: even the a priori forms that can let us know anything in accordance with reason, prohibit us from absolutely knowing things in themselves.

At the end of the first division, one finds a section from Kant which frees his doctrine from any vestiges of subjective idealism which would deny the existence of external objects. Kant does nothing except distinguish the phenomenon from the object. This does not declare that nothing exists apart from itself or of its own consciousness, as a subjective idealist would, but rather declares that knowledge is limited to phenomena and cannot broach the study of things in themselves. In addition, he makes explicit his refutation of idealism in the fourth paralogism, titled "Of ideality (in regard to outer relations)". Attempting to distinguish his position from this subjective idealism, he defines his position as a transcendantal idealism in accord with an empirical realism.

Transcendental Logic

The Transcendental Logic is that part of the CPR where Kant investigates the understanding and its role in constituting our knowledge. The understanding is defined as the faculty of the mind which deals with concepts (A51-52/B75-76). The Logic is divided into two parts: the Analytic and the Dialectic. In the Analytic Kant investigates the contributions of the understanding to knowledge. In the Dialectic Kant investigates the limits of the understanding.

The idea of a transcendental logic is that of a logic which gives an account of the origins of our knowledge as well as its relationship to objects. This is contrasted by Kant with the idea of a general logic, which abstracts from the conditions under which our knowledge is acquired, and from any relation that knowledge has to objects.

Kant's investigation resulted in his claim that the real world of experience can only be an appearance or phenomenon. What things are in themselves, or, other than being appearances, are completely unknowable by any animal or human mind.

Transcendental Analytic

The Transcendental Analytic is divided into an Analytic of Concepts and an Analytic of Principles, as well as a third section concerned with the distinction between phenomena and noumena. The main sections of the Analytic of Concepts are The Metaphysical Deduction and the Transcendental Deduction of the Categories. The main sections of the Analytic of Principles are the Schematism, Axioms of Intuition, Anticipations of Perception, Analogies of Experience, Postulates and The Refutation of Idealism.

The Metaphysical Deduction

In the Metaphysical Deduction Kant aims to derive the pure concepts of the understanding (what he also calls "categories") from the logical forms of judgment. In the Metaphysical Deduction Kant introduces a table of judgments which he uses to guide the derivation of the table of categories.

He creates a list of categories by first enumerating the forms of possible objective empirical judgment which are endowed with their objectivity in virtue of the a priori concepts inherent. Kant’s claim is that if we can identify all of the possible forms of objective empirical judgment, we can then hope to use them as the basis to discover all of the most general concepts or categories that are employed in making such judgments, and thus that are employed in any cognition of objects.

Kant begins from Aristotelian logic in defining four aspects in which one can classify any judgment: quantity, quality, relation, or modality. In each of these ‘moments’ of judgment, there are three alternative classifications; quantity, a judgment may be universal, particular, or singular; relation, a judgment may be categorical, hypothetical, or disjunctive, etc. These Aristotelian ways of classifying judgments are the basis for his discerning the twelve correlated concepts of the understanding. Kant ultimately distinguishes twelve pure concepts of the understanding divided into four classes of three:

Quantity

Unity

Plurality

Totality

Quality

Reality

Negation

Limitation

Relation

Inherence and Subsistence (substance and accident)

Causality and Dependence (cause and effect)

Community (reciprocity between agent and patient)

Modality

Possibility--Impossibility

Existence--Non-existence

Necessity--Contingency

The Transcendental Deduction

In the Transcendental Deduction, Kant aims to show that the categories derived in the Metaphysical Deduction are conditions of all possible experience. He achieves this proof by roughly the following line of thought: all representations must have some common ground if they are to be the source of possible knowledge (because extracting knowledge from experience requires the ability to compare and contrast representations that may occur at different times or in different places), this ground of all experience is the self-consciousness of the experiencing subject, and the constitution of the subject is such that all thought is rule-governed in accordance with the categories. It follows that the categories feature as necessary components in any possible experience.

The Schematism

The 12 categories are related to phenomenal appearances through schemata. Each category has a schema. It is a connection through time between the category, which is an a priori concept, and a phenomenal a posteriori appearance. These schemata are needed to link the pure categories to sensed phenomenal appearances because the categories are, as Kant says, completely heterogenous with sense intuition.

The Refutation of Idealism

In order to answer criticisms of the Critique of Pure Reason that Transcendental Idealism denied the reality of external objects, Kant added a section to the second edition (1787) entitled "The Refutation of Idealism" that turns the "game" of idealism against itself by arguing that self-consciousness presupposes external objects in space. Defining self-consciousness as a determination of the self in time, Kant argues that all determinations of time presuppose something permanent in perception and that this permanent cannot be in the self, since it is only through the permanence that one's existence in time can itself be determined. This argument inverted the supposed priority of inner over outer experience that had dominated philosophies of mind and knowledge since Descartes.

Transcendental Dialectic

Following the systematic treatment of a priori knowledge given in the transcendental analytic, the transcendental dialectic seeks to dissect dialectical illusions. Its task is effectively to expose the fraudulence of non-empirical employment of the understanding.

This longer but less dense section of the Critique is composed of five essential elements, as follows:

Introduction (to Reason and the Transcendental Ideas)
Rational Psychology (the nature of the soul)
Rational Cosmology (the nature of the world)
Rational Theology (God)
Appendix (on the consitutive and regulative uses of reason)

In the introduction, Kant introduces a new faculty, human reason, positing that it is a unifying faculty which unifies the manifold of knowledge gained by the understanding. Another way of thinking of reason is to say that it searches for the 'unconditioned'; Kant had shown in the Second Analogy that every empirical event has a cause, and thus each event is conditioned by something antecedent to it, which itself has its own condition, and so forth. Reason seeks to find an intellectual resting places which may bring the series of empirical conditions to a close, to obtain knowledge of an 'absolute totality' of conditions, thus becoming unconditioned.

Mediate representations represent things by representing general characteristics of things. Immediate representations represent thing directly. For example, consider a particular chair. The concepts "brown," "wooden," "chair," and so forth are, according to Kant, mediate representations of the chair. They can represent the chair by representing general characteristics of the chair: being brown, being wooden, being a chair, and so forth. One's perception of the chair, however, is, according to Kant, an immediate representation. The perception represents the chair directly, and not by means of any general characteristics.

Kant divides intuitions into groups in several different ways. First, Kant distinguishes intuitions into pure intuitions and empirical intuitions. Empirical intuitions are intuitions that contain sensation. Pure intuitions are intuitions that do not contain any sensation (A50/B74). An example of an empirical intuition would be one's perception of a chair or other physical object. All such intuitions are immediate representations that have sensation as part of the content of the representation. The pure intuitions the human mind possesses are, according to Kant, those of space and time. Our representations of space and time are immediate representations, and do not include sensation within those representations. Thus both are pure intuitions.

Kant also divides intuitions into two groups in another way. Some intuitions require the presence of their object, i.e. of the thing represented by the intuition. Other intuitions do not. (The best source for these distinctions is Kant's Lectures on Metaphysics.) We might think of these in non-Kantian terms as first, perceptions, and second, imaginations (see B151). An example of the former: one's perception of a chair. An example of the latter: one's memory of building that has subsequently been destroyed. Throughout the Transcendental Aesthetic, Kant seems to restrict his discussion to intuitions of the former type: intuitions that require the presence of their object.